1. Field of the Invention
The present invention relates to a method of driving a piezoelectric actuator including a piezoelectric body which has respective coercive electric field points on a negative field side and a positive field side, and has asymmetrical bipolar polarization/field hysteresis characteristics in which an absolute value of the coercive electric field on the negative field side and the coercive electric field value on the positive field side are mutually different, and also relates to a piezoelectric actuator used in this drive method, a liquid ejection apparatus using this piezoelectric actuator, and a piezoelectric type ultrasonic oscillator.
2. Description of the Related Art
A piezoelectric element which comprises a piezoelectric body having piezoelectric characteristics whereby the body expands and contracts with increase and decrease in the intensity of an applied electric field, and an electrode which applies an electric field to the piezoelectric body, is used in applications such as piezoelectric actuators mounted in inkjet type recording heads. For the piezoelectric material, a perovskite type oxide is commonly used, and of this, a lead titanate zirconate (PZT) type of perovskite oxide is known to have high piezoelectric characteristics (piezoelectric d constant).
The bipolar polarization/field curve (P-E hysteresis) of a general piezoelectric body is substantially symmetrical with respect to the y axis which indicates the polarization value, and the coercive electric field Ec1 on the negative field side and the coercive electric field Ec2 on the positive field side are substantially equal (|Ec1|≈Ec2).
On the other hand, the P-E hysteresis of a piezoelectric film, or the like, made of a PZT type perovskite oxide which is deposited by a vapor deposition method such as sputtering, is biased toward the positive field side, in other words, is asymmetrical with respect to the y axis, and the absolute value of the coercive electric field Ec1 on the negative field side and the coercive electric field Ec2 on the positive field side are different (|Ec1|≠Ec2) (see FIG. 1A). A piezoelectric body which displays P-E hysteresis characteristics of this kind produces a very large polarization upon application of a slight negative voltage during negative driving, but only produces slight polarization during positive driving, even when applying a positive voltage having the same absolute value as the negative voltage that has been applied.
As described above, in a piezoelectric body having P-E hysteresis which is asymmetrical with respect to the y axis, it is possible to obtain high piezoelectric performance by actively utilizing this asymmetry, in other words, by driving the body by applying an electric field of the polarity where the absolute value of the coercive electric field is smaller. A piezoelectric actuator having a piezoelectric body of this kind is desirable from the viewpoint of being able to yield large displacement, but in order to obtain a large displacement by means of a normal method of driving a piezoelectric actuator which is driven from zero within the range of the electric field Emax (0≦E≦|Emax|) which indicates the maximum displacement on the electric field side of the polarity having the smaller absolute value of the coercive electric field, it is necessary to apply a large electric field (voltage), and hence there are possibilities that the load on the piezoelectric body is large, and the drive durability and the element reliability becomes poor.
Japanese Patent Application Publication No. 2003-243741 and Japanese Patent Application Publication No. 2008-218547 disclose a piezoelectric element which actively utilizes the asymmetrical P-E hysteresis characteristics. Japanese Patent Application Publication No. 2003-243741 discloses a piezoelectric body having P-E hysteresis characteristics in which two coercive electric field points are both situated on the same field polarity.
If a piezoelectric body having coercive electric fields Ec1 and Ec2 with opposite polarities is driven by a single polarity in this way, by the normal piezoelectric actuator driving described above (0≦E≦|Emax|), then the displacement on the opposite side to the driving polarity is not used and creates loss; however, in the piezoelectric body described in Japanese Patent Application Publication No. 2003-243741, the two coercive electric field points are situated on the same polarity, and therefore it is possible to make effective use of displacement that is lost in a normal piezoelectric body.
Furthermore, Japanese Patent Application Publication No. 2008-218547 discloses a piezoelectric body having asymmetrical P-E hysteresis, and materials and design for same, the piezoelectric body having hysteresis characteristics whereby the drive voltage and the gradient of the displacement become a maximum, thus obtaining a piezoelectric body having low displacement loss and low power loss and excellent durability with long use compared to the piezoelectric body described in Japanese Patent Application Publication No. 2003-243741.
However, in Japanese Patent Application Publication No. 2003-243741 and Japanese Patent Application Publication No. 2008-218547, the material is designed to improve the P-E hysteresis and field-distortion curve characteristics, in such a manner that a large displacement is obtained efficiently. Consequently, Japanese Patent Application Publication No. 2003-243741 and Japanese Patent Application Publication No. 2008-218547 do not reduce the load on the piezoelectric body by efficient driving of a currently available piezoelectric body having asymmetrical P-E hysteresis, such as PZT or the like.
On the other hand, International Publication WO 2003/022582 discloses a liquid ejection apparatus which applies a drive waveform including a high potential period during which a voltage producing an electric field intensity exceeding the coercive electric field of the piezoelectric body is applied, and an inverse potential period during which a voltage of inverse polarity to the voltage of the high potential period is applied. International Publication WO 2003/022582 discloses, in FIG. 9 and the explanation relating to FIG. 9 (the first embodiment from page 8 to page 9), that, by applying a drive waveform having an inverse potential period, a large displacement is obtained, even with the same amount of change in the electric field, and furthermore that sufficient displacement can be obtained without being susceptible to the effects of change in the hysteresis curve due to the multiple driving actions. However, International Publication WO 2003/022582 does not describe the application of a piezoelectric actuator comprising a piezoelectric body having asymmetrical P-E hysteresis.